Various types of batteries have been heretofore developed, and in every battery, a packaging material is a member that is absolutely necessary for encapsulating battery elements such as an electrode and an electrolyte. Metallic packaging materials have been often used heretofore as battery packages, but in recent years, batteries have been required to be diversified in shape, and desired to be thinner and lighter as performance of electric cars, hybrid electric cars, personal computers, cameras and mobile phones has been enhanced. However, metallic battery packaging materials that have been often used heretofore have the disadvantage that it is difficult to keep up with diversification of shapes, and there is a limit to weight reduction.
Thus, in recent years, there has been proposed a film-shaped laminate with a base material layer, an adhesive layer, a metal layer and a sealant layer laminated in this order has been proposed as a battery packaging material which is easily processed into diversified shapes and is capable of achieving thickness reduction and weight reduction (see, for example, Patent Document 1). Such a film-shaped battery packaging material is formed so as to be able to encapsulate a battery element by heat-sealing a peripheral edge by heat sealing with sealant layers facing each other.
On the other hand, the battery, depending on a type of electrolyte, may generate a combustible gas, leading to an increase in pressure. For example, when the battery is exposed to a high temperature, an organic solvent used in an electrolytic solution may be decomposed to generate a combustible gas, leading to an increase in pressure. The battery may be charged with an excessive voltage or discharged at an excessive current to persistently increase the temperature in the battery, leading to uncontrollable battery reaction.
In a battery using a film-shaped battery packaging material, an increase in pressure or temperature in the battery may cause cleavage of the battery packaging material, leading to occurrence of firing or the like due to eruption of a combustible gas. When the pressure or temperature in the battery persistently increases, so that the battery reaction becomes uncontrollable while the battery packaging material is excessively expanded, the battery may be exploded.
As a battery packaging material capable of suppressing cleavage at a heat-sealed part, and occurrence of breakage immediately before the heat-sealed part even when the pressure in a battery persistently increases, one including, on a sealant layer or an adhesive resin layer adjacent thereto, a cleavage induction portion at which stress during cleavage is smaller than stress during cleavage of the sealed surfaces of sealant layers has been presented (see Patent Document 2). In Patent Document 2, however, merely progress of cleavage with low stress at the interface between the adhesive resin layer and a corrosion inhibition treatment layer on a metal layer is suppressed by inducing cleavage to the cleavage induction portion, but the battery packaging material is not designed so that the battery can be gently unsealed at the time when the battery turns into a state in which the pressure or temperature in the battery persistently increases. Actually, in Patent Document 2, the battery packaging material is designed so that when strong stress is applied, interlayer delamination in which cleavage progresses between adhesive layers, or cohesive fracture in which cracking progresses in an adhesive layer occurs. The risk of firing, explosion or the like due to rapid eruption of a combustible gas may be increased if the battery is unsealed due to interlayer delamination or cohesive fracture at the time when the pressure or temperature in the battery increases.
Thus, for ensuring safety when the pressure or temperature in the battery persistently increases, the battery packaging material is required to be designed so that until the ambient temperature reaches a certain temperature, the battery packaging material is not cleaved, and a battery element is kept hermetically sealed to suppress firing etc. resulting from rapid eruption of a combustible gas, and thereafter the battery is gently unsealed to slowly release a gas in the battery packaging material.